Journal of Experimental Botany, Vol. 51, No. 90001, pp. 339-346,
February 2000
© 2000 Oxford University Press
Control of C4 photosynthesis: effects of reduced activities of phosphoenolpyruvate carboxylase on CO2 assimilation in Amaranthus edulis L.
1 Robert Hill Institute and Department of Animal and Plant Sciences, University of Sheffield, Sheffield S10 2TN, UK
2 Istituto l'Agroselvicoltura, via Marconi 2, 05010 Porano, Italy
3 Department of Biological Sciences, Lancaster University, Lancaster LA1 4YQ, UK
Received 19 April 1999; Accepted 4 August 1999
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and methodsResultsReferences |
|---|
Heterozygous mutants of Amaranthus edulis deficient in PEP carboxylase (PEPC) have been used to study the control of photosynthetic carbon assimilation. A reduction in PEPC activity led to a decrease in the initial slope of the relationship between the CO2 assimilation rate and the intercellular CO2 concentration and to a decrease in photosynthesis at high light intensities, consistent with a decrease in the capacity of the C4 cycle in high light. PEPC exerted appreciable control on photosynthetic flux in the wild-type, with a relatively high flux control coefficient of 0.35 in saturating light and ambient CO2. The flux control coefficient was decreased in low light or increased in low CO2 or in plants containing lower PEPC activity. However, the rate of CO2 assimilation decreased down to about 55% PEPC, followed by an up-turn in the light-saturated photosynthetic rate as PEPC was further reduced, suggesting the existence of a mechanism that compensates for the loss of PEPC activity. The amounts of photosynthetic metabolites, including glycine and serine, also showed a biphasic response to decreasing PEPC. There was a linear relationship between the activity of PEPC and the activation state of the enzyme. A possible mechanism of compensation involving photorespiratory intermediates is discussed.
Key words: C4 plants, Amaranthus edulis, phosphoenolpyruvate carboxylase, CO2 assimilation, control of photosynthesis.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsReferences |
|---|
C4 plants suppress photorespiration by concentrating CO2 at the active site of Rubisco. This is done by means of a CO2 pump in which CO2 is assimilated by phosphoenolpyruvate carboxylase (PEPC) in the mesophyll cells into C4 acids that are transferred to, and decarboxylated in, the thick-walled bundle-sheath cells. Like many enzymes involved in photosynthetic CO2 assimilation, PEPC is highly regulated to accommodate large changes in photosynthetic fluxes and to allow co-ordination with other enzymes involved in CO2 fixation and carbohydrate synthesis (Leegood et al., 1997). This regulation takes two main forms. First, the enzyme is very responsive to metabolite effectors, such as inhibition by malate and activation by sugar phosphates (Ting and Osmond, 1973; Doncaster and Leegood, 1987). Second, the effects of metabolites are modulated by phosphorylation of the enzyme. Reversible, light-dependent, phosphorylation of PEPC occurs in C4 plants, which leads to activation by a decreased sensitivity to inhibition by malate (Huber et al., 1994; Chollet et al., 1996) and increased sensitivity to activation by glucose-6-P (Duff et al., 1995). The mechanisms which regulate the activity of the PEPC kinase have yet to be fully established (Huber et al., 1994; Chollet et al., 1996).
Recently, a number of mutant and transgenic C4 plants have been isolated which are deficient in various enzymes of photosynthetic carbon metabolism (Dever et al., 1995; Furbank et al., 1997). One of these plants is a mutant of the grain amaranth, Amaranthus edulis, that lacks the C4 isoenzyme of PEPC. Without the operation of the CO2 pump, homozygous PEPC mutants scarcely grow in air (as they are at around the CO2 compensation point), but they grow satisfactorily in elevated concentrations of CO2, which can then diffuse into the bundle-sheath at concentrations adequate to support net photosynthesis. Heterozygous plants with half the PEPC activity of the wild type grow normally in air and are similar in appearance to the wild type (Dever et al., 1995, 1997). In this study, heterozygous plants containing varying amounts of PEPC have been generated in order to quantify the control that it exerts over photosynthetic CO2 fixation in C4 plants. In addition, an attempt has been made to establish whether or not changes in regulatory metabolites and the activation state of PEPC can compensate for the loss of PEPC activity which occurs in heterozygous plants.
|
Materials and methods |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsReferences |
|---|
Plant material and growth conditions
A mutant of amaranthus (Amaranthus edulis Speg.) deficient in PEPC (LaC4 2.16; Dever et al., 1995) was crossed with a wild-type plant and the F1 progeny were self-fertilized. Plants of the F2 generation (and wild-type plants from separate seed) were grown in a 2 : 1 mixture of vermiculite and high nutrient compost (M3; Fisons, Ipswich, UK) in a greenhouse (maximum air temperatures 31 °C, minimum 16 °C) at a PFD which varied between 300 and 1800 µmol m-2 s-1 (between April and August). Plants were provided with a nutrient solution (Miracle-Gro; ICI, Haslemere, UK) twice a week. Plants were sampled between 8 and 15 weeks. Leaf material was frozen in the light in liquid N2 between 12.30 h and 14.30 h at PFDs of 1000–1500 µmol quanta m-2 s-1 and stored either in liquid N2 or at -80 °C.
Measurement of PEP carboxylase
Leaf discs (1.33 cm2) were extracted (Ashton et al., 1990) in 1 ml of ice-cold 50 mM MOPS-KOH (pH 7.2), 10 mM MgCl2, 1 mM DTT, 10% (v/v) glycerol, and 2% (w/v) BSA, using an ice-cold pestle and mortar and centrifuged for 2 min at 14 000 g. The activity was assayed within 60 s at 25 °C in 25 mM TRIS-HCl (pH 8.0), 5 mM MgCl2, 2 mM DTT, 1 mM KHCO3, 5 mM Glc6P, 5 mM PEP, 0.2 mM NADH, 2 U ml-1 MDH, and 5 µl of extract.
Activation state of PEP carboxylase
Frozen leaf discs (1.33 cm2) were extracted in 100 mM HEPES-KOH (pH 8.0), 10% glycerol, 5 mM MgCl2, 2 mM EGTA, 5 mM DTT, 20 mM NaF, 1 µg ml-1 chymostatin, and 20 µl of Complete Protease Inhibitor Cocktail (Boehringer Mannheim, Mannheim, Germany) (1 tablet in 2 ml H2O), in an ice-cold mortar and pestle with a small quantity of acid-washed sand. Extracts were centrifuged for 2 min at 14 000 g and assayed immediately in 100 mM HEPES (pH 7.3), 5 mM MgCl2, 4.8 mM NaHCO3, 0.35 mM NADH, 5 U ml-1 MDH, 0.8 mM PEP, and 100 µl of extract at 25 °C.
SDS-PAGE and immunoblotting
SDS-PAGE and Western immunoblotting were done as previously described (Walker et al., 1997). For dot blots, leaf discs (1.33 cm2) were homogenized in 0.3 ml extraction buffer containing 180 mM Bicine (pH 9.0), 18 mM MgCl2, 10% SDS, and 5 mM DTT, with a small quantity of acid-washed sand, in a cold mortar. After centrifugation for 15 min at 14 000 g, extracts were boiled for 1 min with equal volumes of solubilization buffer (62.5 mM TRIS, 20% (v/v) glycerol, 2.5% (w/v) SDS, and 5% (v/v) 2-mercaptoethanol, pH 6.8). Sample volumes of 1 µl (three replicates each of five dilutions) were spotted onto a polyvinylidene difluoride (Immobilon-P) membrane and probed with antisera raised in rabbit against PEPC from A. edulis, and for Rubisco from rape (from Dr Martin Parry, Rothamsted Experimental Station, UK), as well as antibodies against maize pyruvate, Pi dikinase (from Professor R Chollet, University of Nebraska, USA). The membrane was incubated for 20 min with a 1 : 500 dilution of a 35S-labelled anti-rabbit secondary antibody (raised in donkey) (Amersham International plc, Little Chalfont, UK). After incubation, the membrane was washed five times in TBS and radioactivity measured by scintillation counting.
Determination of NAD-malic enzyme activity
All enzyme assays were performed at 25 °C. Leaf discs (1.33 cm2) were extracted in 1 ml of ice-cold 25 mM HEPES-KOH (pH 7.5), 25 mM Tricine-KOH (pH 7.5), 5 mM DTT, 2 mM MnCl2, 0.5% (v/v) Triton X-100, and 0.25% (w/v) PVP-40, using an ice-cold pestle and mortar and assayed at 25 °C (according to Ashton et al., 1990).
Determination of metabolites
Leaf samples (c. 5 cm2) were extracted in HClO4 and assayed for metabolites as described previously (Lowry and Passonneau, 1972). Recoveries of all metabolites were between 80% and 100% of the expected values. Amino acids were quantified by separation of the o-phthaldehyde derivatives on a Spherisorb ODS2 5 µm HPLC column (Fleury and Ashley, 1983).
13C analysis
Leaf discs (1.33 cm2) were dried at 70 °C and analysed by a Roboprep Dumas combustion unit coupled to a Tracermass isotope ratio mass spectrophotometer (Europa Scientific Ltd., Crewe, UK). A 13C value of -8
was assumed for atmospheric CO2.
Control coefficients
The control coefficient (C), was calculated from the slope (ab/(E+b)2) of a rectangular hyperbolic function (aE/(b+E)) at any E, and multiplied by a scaling factor (E/J), where a and b are constants, E is the amount of enzyme and J is the flux through the pathway at E (Quick, 1994).
Gas exchange measurements
CO2 assimilation rates were measured with an open infrared gas analyser system (LCA4, ADC, Hoddesdon, UK) at a temperature of between 26 °C and 32 °C and a relative air humidity of 40–50% using KL 1500 light source (Schott, Mainz, Germany). Intercellular CO2 concentrations (Ci) were calculated according to von Caemmerer and Farquhar (von Caemmerer and Farquhar, 1981).
Protein and chlorophyll
Total protein was assayed according to Bradford (Bradford, 1976). Chlorophyll was extracted in 96% (v/v) ethanol and quantified according to Lichtenthaler and Wellburn (Lichtenthaler and Wellburn, 1983).
|
Results |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsReferences |
|---|
Production of heterozygous plants
Over 100 F2 plants and 12 wild-type plants, each from separate batches of seed, grown in air for 4 weeks, were screened for PEPC activity in 1995. For experimental work in subsequent years (1996 and 1997) seed was collected from selfed heterozygotes. In 1995, 21 plants were selected and placed in seven groups which contained reduced PEPC activities in a range between 45% and 76% of the wild type (Table 1
). For the plants selected, PEPC activities were monitored over a period of 3 months. No appreciable changes in the relative activities of PEPC were detected. Although heterozygous plants would be expected to have 50% of the wild-type activity, the range encountered was probably also a result of the variation in PEPC activity observed in the wild-type population. Quantification of the amount of PEPC protein from Western blots was found to be unreliable due to incomplete transfer of PEPC to the nitrocellulose membrane. Dot blots were therefore employed to quantify PEPC. There was a linear relationship between the amount of PEPC protein and its maximum catalytic activity (Fig. 1).
|
|
Pleiotropic effects
There were no obvious differences in growth or appearance between any of the selected range of heterozygous and wild-type plants in years 1–3. Table 1 shows that a reduction of PEPC activity down to 45% had no significant impact on the leaf content of soluble protein, chlorophyll content, chlorophyll a/b ratio or the activities or amounts extracted from leaves of a number of enzymes of carbon metabolism, including NAD-malic enzyme, Rubisco and pyruvate, Pi dikinase (Table 1). Similar results were obtained in two successive years. There was no evidence for changes in the distribution of Rubisco and PEPC between the mesophyll
and bundle-sheath (estimated by immunolocalization in tissue sections), for changes in the carbon isotope ratio of leaf material, which lay between -12.64 (66% PEPC) and -13.35 (wild type), in the size of the mesophyll cells or in the interveinal distance (data not shown).
Control of photosynthesis
The relationship between the CO2 assimilation rate (A) and the intercellular concentration of CO2 (Ci) was measured in each of the groups of plants. There was a decrease in the initial slope of the A/Ci curves of the heterozygous plants (Fig. 2 and data not shown). The intercellular CO2 concentration at which CO2-assimilation was saturated was also increased in all of the heterozygote groups compared to the wild-type, for example, in the wild-type saturation occurred at a Ci of 107 µl l-1 whereas in the 55% mutant group it was at 130 µl l-1. These data indicate a decline in the carboxylation efficiency due to a reduction in the amount of PEPC. In all of the heterozygous plant groups there was no appreciable change in the CO2 compensation point compared to the wild type, suggesting that PEPC was responsible for CO2 fixation rather than direct fixation by Rubisco and that there was no photorespiratory loss of CO2. The response of the CO2 assimilation rate to PFD was measured at ambient CO2 (Fig. 3). There was a reduction in the assimilation rate in heterozygous plants at higher PFDs when compared to the wild type. At low and moderate PFDs (values below 360 µmol m-2 s-1) the response of the assimilation rate for both wild-type and heterozygote plants was similar.
|
) and wild-type (•) plants at a PFD of 1200 µmol quanta m-2 s-1. Each data point is the mean of three (mutant) and six (wild-type) plants ±SE..
|
The response of the assimilation rate to various intercellular CO2 concentrations at saturating PFD was analysed in the range of heterozygous and wild-type plants (Fig. 4
). Two points should be noted. First, as the CO2 concentration was reduced, the decrease in the CO2 assimilation rate became steeper with a decrease in PEPC. Second, at PEPC activities below 55%, there was a surprising upturn in the rate of CO2 assimilation. This upturn in plants with below 55% wild-type PEPC was significant compared to plants with 55% PEPC (P<0.05) and was observed in each of the three years of study in separetely generated groups of heterozygous plants.
|
) and the 55, 70 and 100% (•) mutant plants..The degree of control exerted by PEPC on photosynthetic CO2 assimilation was estimated by using the principles of control analysis (Kacser and Burns, 1973). It is clear that at low (30 µl l-1) CO2 concentrations the flux control coefficient, CJc, in the wild-type was higher than at a Ci of 60 µl l-1, which, in turn, was higher than at an ambient Ci of 130 µl l-1 (Fig. 4). At ambient Ci, CJcPEPC increased from 0.35 in the wild type, to 0.49 at 55% PEPC. At moderate CO2, CJcPEPC increased from 0.65 in the wild type to 0.77 at 55% PEPC. At low CO2 CJcPEPC in the wild type was 0.70, increasing to 0.81 at 55% PEPC.
Changes in activation state
The apparent phosphorylation state of PEPC was measured by its sensitivity to malate (McNaughton et al., 1991). Preliminary experiments demonstrated that the largest difference in malate sensitivity between the wild type and heterozygous plant groups was obtained from samples harvested in the middle of the day. There was no appreciable increase in malate sensitivity in extracts from leaves subject to a period of more than 2 h dark adaptation. Measurements of maximum PEPC activity demonstrated that there were no significant differences in the maximum activity of PEPC between light or dark samples (data not shown).
Figure 5 shows the malate sensitivities for the range of heterozygous and wild-type plants measured for extracts from both light- and dark-adapted leaves. The concentration of malate was 0.3 mM. Similar results were obtained at a concentration of 0.5 mM malate. In light-adapted extracts there was a linear relationship between the PEPC activity and the apparent phosphorylation state of PEPC, measured by its sensitivity to malate. The percentage inhibition of PEPC decreased in a broadly proportional manner with a decrease in PEPC, suggesting an increase in the apparent phosphorylation state of PEPC with reduced PEPC amount. The dark-adapted leaf extracts from both the heterozygous and wild-type plants showed a very similar high sensitivity to malate and inhibition of PEPC indicative of the less phosphorylated form of the enzyme.
|
Amounts of metabolites
There was a decline in the amounts of the metabolites hexose-P, triose-P, Fru1,6-bisP, glycerate-3-P, and RuBP down to 55% of the wild-type amount of PEPC (Fig. 6), but as with rates of photosynthesis (Fig. 4), a further decrease in the amount of PEPC below 55% of the wild type, led to an increase in the amounts of these metabolites. There was no significant change between the amounts of PEP and pyruvate between the wild type and the heterozygous plants. For the amino acids, aspartate and alanine, there was an increase in amount from the wild type as PEPC decreased. In the case of serine, and particularly glycine, there was also a significant upturn in the amounts as PEPC was decreased below 55% of the wild type (P<0.05 compared with plants with 55% PEPC).
|
Discussion
Plants with a range of PEPC activities were stable relative to one another during the duration of the experiments. There was no evidence for pleiotropic effects on chlorophyll, protein, a range of other enzymes or on leaf anatomy (see also Dever et al., 1995).
Control of photosynthesis
The relationship between the CO2 assimilation rate (A) and the intercellular CO2 concentration (Ci) and the PFD in wild-type and heterozygous plants showed that the initial slope (the carboxylation efficiency) decreased linearly with a reduction in PEPC down to 55% of the wild-type amount (data not shown). The initial slope of the A/Ci curve in a C4 plant is believed to reflect the kinetic characteristics and activity of PEPC (Collatz et al., 1992) and similar effects are seen when an inhibitor of PEPC, DCDP, is fed to intact leaves (Jenkins et al., 1989). A decrease in the capacity of PEPC will lead to a reduction in the amount of CO2 in the bundle sheath. The effect of a reduction in PEPC on the CO2 assimilation rate was also dependent upon PFD, with an appreciable decrease in photosynthesis at PFDs higher than about 360 µmol m-2 s-1, but the light response curves were similar for the wild-type and range of heterozygous plants at lower PFDs. These data are consistent with a decrease in the capacity of C4 cycle in high light and with the view that, in low light, C4 photosynthesis is limited by RuBP or PEP regeneration (Collatz et al., 1992; Furbank et al., 1996; Trevanion et al., 1997; von Caemmerer and Furbank, 1999).
PEPC exerted appreciable control on photosynthetic flux in the wild type, with a relatively high CJcPEPC of 0.35 in saturating light and ambient CO2. Control was decreased in low light and increased in low CO2 or in plants with less PEPC. The results presented in Fig. 5 also indicate a compensatory increase in activation state as a consequence of reduced PEPC activity. Similar results have been obtained for a range of other enzymes. However, it is clear that, in the case of enzymes such as NADP-malate dehydrogenase in the C4 plant, Flaveria bidentis, and phosphoribulokinase in tobacco, the combination of thioredoxin-mediated activation and metabolite modulation is sufficient both to compensate for loss of enzyme activity and also to remove most, if not all, of the ability of the enzyme to control flux (Paul et al., 1995; Trevanion et al., 1997). Many enzymes which exhibit high regulatability and catalyse non-reversible reactions have low control coefficients (Stitt, 1995). In contrast, loss of PEPC protein cannot be fully compensated by activation of PEPC by either metabolites or by covalent modification. This then raises questions as to the function of the phosphorylation-dependent activation of PEPC. Phosphorylation is insufficient to compensate fully for loss of enzyme activity, so it seems unlikely that it acts in a dynamic manner like the thioredoxin-mediated regulation of enzymes of the Benson–Calvin cycle, which ensures that all the enzymes in that pathway respond rapidly to increases in flux brought about by changes in light intensity (Fell, 1997). This view is supported by the observation that phosphorylation and dephosphorylation are slow (minutes to hours; McNaughton et al., 1991; Duff et al., 1995), compared with the rapid modulation of other enzymes of the Benson–Calvin cycle and C4 cycle, which occurs within minutes or seconds (Edwards et al., 1985; Leegood et al., 1985). It seems much more likely that phosphorylation of PEPC acts as a general ‘on-off’ switch over a much longer time-scale, or that it may play some role in damping the response of PEPC to fluctuations in metabolite effectors which may occur during, for example, fluctuations in light intensity (Doncaster et al., 1989).
In C4 plants, a number of enzymes with high regulatability appear to exert appreciable control over the photosynthetic CO2 flux. In F. bidentis the control exerted by pyruvate, Pi dikinase and by Rubisco was appreciable, with a wild-type CJcPPDK of between 0.2 and 0.3 and CJcRubisco between 0.5 and 0.7 (Furbank et al., 1997) under broadly similar conditions (high PFD and ambient CO2). It would thus appear that Rubisco and the enzymes of the C4 cycle predominate in the control of C4 photosynthesis.
Possible compensatory mechanisms for a reduction in PEPC activity
In each of the three years, a clear trend emerged in which the CO2 assimilation rate decreased down to about 55% PEPC, followed by an up-turn in the light-saturated photosynthetic rate. These findings suggest that a compensation mechanism begins to operate once the amount of PEPC drops below a critical amount. A similar response has been seen in the release of ammonia in barley plants with decreased amounts of glutamine synthetase (Häusler et al., 1996).
There are a number of possibilities which might explain how compensation of the loss of PEPC protein could occur. The first is that compensation is achieved by a reversion of these plants into a mode of photosynthesis more like that of a C3 plant. This is not supported by the anatomical studies or by measurements of carbon isotope discrimination. A second is that an enzyme substitutes for PEPC in the C4 cycle. The only possibility is PEP carboxykinase, but wild-type A. edulis lacks this enzyme (Walker et al., 1997) and PEP carboxykinase has a notably low affinity for CO2 (Urbina and Avilan, 1989). Another possibility is that the compensation mechanism involves an activation of PEPC, which could occur either by an increase in activating metabolites, or a decrease in inhibitory metabolites, or else by a change in the activation state of the enzyme, brought about by increased phosphorylation of PEPC, either triggered by factors which activate PEPC kinase, such as glycerate-3-P, or changes in cytosolic pH or Ca2+ (Chollet et al., 1996) or by increases in the amount of the kinase itself.
There was a linear relationship between the activity of PEPC and the activation state of the enzyme, as estimated by its sensitivity to malate. Changes in the activation state of PEPC (and, by inference, changes in the activity of PEPC kinase) cannot, therefore, explain the biphasic change in CO2-assimilation rate. The amounts of phosphorylated metabolites, the C4-cycle intermediates, aspartate and alanine, and the photorespiratory intermediates, glycine and serine, broadly followed the photosynthetic assimilation rate (Leegood and von Caemmerer, 1988, 1989) and, with the exception of PEP and pyruvate, showed a biphasic response to decreasing PEPC. The question which is then raised is whether or not the increase in photosynthesis caused the rise in the amounts of phosphorylated metabolites and amino acids, or conversely, if the increase in these metabolites caused the subsequent increase in photosynthesis, perhaps by acting in their capacities as allosteric effectors of PEPC. There can be little doubt that the increases in the amounts of positive effectors such as triose-P, Glc6P, Fru6P, glycine, and serine which are observed below c. 55% PEPC will tend to activate PEPC allosterically (Doncaster and Leegood, 1987). However, it is also clear that these changes are accompanied by increases in negative effectors, such as aspartate and glutamate and, presumably malate (Doncaster and Leegood, 1987; O'Leary, 1982). However, one feature that is different is the rise in glycine and serine. The amounts of glycine and serine are likely to be indicative of the rate of photorespiration (Leegood and von Caemmerer, 1994; Dever et al., 1995). Both are activators of PEPC in C4 plants (Uedan and Sugiyama, 1976; O'Leary, 1982; Doncaster and Leegood, 1987; Gao and Woo, 1996), including A. edulis, in which 5 mM glycine and serine activated PEPC by 27% and 23%, respectively (data not shown). It is likely that, as photorespiratory rates increase at lower bundle sheath CO2 concentrations, the concentrations of glycine and serine will rise and they will leak out of the bundle sheath. These will increase CO2 pumping by the C4 cycle by activating PEPC, thereby decreasing photorespiration. However, this feedback mechanism cannot be wholly effective as glycine suddenly increases as PEPC is reduced. A similar sudden rise in glycine occurs in leaves of the C4 plant, Flaveria bidentis, at low CO2 concentrations (Leegood and von Caemmerer, 1994). There is a transition between a point at which the feedback mechanism is effective to one at which it can no longer suppress glycine accumulation. It may be that such a transition to glycine accumulation underlies compensation for the loss of PEPC activity observed in this study.
|
Acknowledgements |
|---|
This research was supported by the Biotechnology and Biological Sciences Research Council through a grant from the Programme on the Biochemistry of Metabolic Regulation in Plants (89/BR301910) and by a research studentship to Karen Bailey. We are grateful to Dr Enrico Brugnoli (Istituto per l'Agroselvicoltura) for use of the mass spectrometer.
|
Footnotes |
|---|
4 To whom correspondence should be addressed. Fax. +44 114 222 0050. E-mail:r.leegood{at}sheffield.ac.uk
|
Abbreviations |
|---|
A, CO2 assimilation rate;; Ci, intercellular CO2 concentration; PEP, phosphoenolpyruvate; PEPC, phosphoenolpyruvate carboxylase; Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase..
|
References |
|---|
|
TopAbstractIntroductionMaterials and methodsResultsReferences |
|---|
Ashton AR, Burnell JN, Furbank RT, Jenkins CLD, Hatch MD. 1990. Enzymes of C4 photosynthesis. In: Lea PJ, ed. Methods in plant biochemistry. New York: Academic Press, 39–72.
Bradford M.1976. A rapid and sensitive method for the quantification of microgram quantities of protein utilizing the principles of protein–dye binding. Analytical Biochemistry 72, 248–254.[Web of Science][Medline]
Chollet R, Vidal J, O'Leary MH. 1996. Phosphoenolpyruvate carboxylase: a ubiquitous, highly regulated enzyme in plants. Annual Review of Plant Physiology and Plant Molecular Biology 47, 273–298.[Web of Science]
Collatz GJ, Ribas-Carbo M, Berry JA. 1992. Coupled photosynthesis-stomatal conductance model for leaves of C4 plants. Australian Journal of Plant Physiology 19, 519–538.
Dever LV, Bailey KJ, Leegood RC, Lea PJ. 1997. Control of photosynthesis in Amaranthus edulis mutants with reduced amounts of PEP carboxylase. Australian Journal of Plant Physiology 24, 469–476.
Dever LV, Blackwell RD, Fullwood NJ, Lacuesta M, Leegood RC, Onek LA, Pearson M, Lea PJ. 1995. The isolation and characterization of mutants of the C4 photosynthetic pathway. Journal of Experimental Botany 46, 1363–1376.[Web of Science]
Doncaster HD, Leegood RC. 1987. Regulation of phosphoenolpyruvate carboxylase activity in maize leaves. Plant Physiology 84, 82–87.
Doncaster HD, Adcock MD, Leegood RC. 1989. Regulation of photosynthesis in leaves of C4 plants following a transition from high to low light. Biochimica et Biophysica Acta 973, 176–184.
Duff SMG, Andreo CS, Pacquit V, Lepiniec L, Sarath G, Condon SA, Vidal J, Gadal P, Chollet R. 1995. Kinetic analysis of the non-phosphorylated, in vitro phosphorylated, and phosphorylation-site-mutant (Asp8) forms of intact C4 phosphoenolpyruvate carboxylase from Sorghum. European Journal of Biochemistry 228, 92–95.[Web of Science][Medline]
Edwards GE, Nakamoto H, Burnell JN, Hatch MD. 1985. Pyruvate, Pi dikinase and NADP-malate dehyrogenase in C4 photosynthesis: properties and mechanism of light/dark regulation. Annual Review of Plant Physiology 36, 255–286.[Web of Science]
Fell D. 1997. Understanding the control of metabolism. London: Portland Press.
Fleury MO, Ashley DV. 1983. High performance liquid chromatographic analysis of amino acids in physiological fluids: on-line precolumn derivatization with o-phthaldehyde. Analytical Biochemistry 133, 330–335.[Web of Science][Medline]
Furbank RT, Chitty JA, Jenkins CLD, Taylor WC, Trevanion SJ, von Caemmerer S, Ashton AR. 1997. Genetic manipulation of key photosynthetic enzymes in the C4 plant Flaveria bidentis. Australian Journal of Plant Physiology 24, 477–485.
Furbank RT, Chitty JA, von Caemmerer S, Jenkins CLD. 1996. Antisense RNA inhibition of RbcS gene expression reduces Rubisco level and photosynthesis in the C4 plant Flaveria bidentis. Plant Physiology 111, 725–734.[Abstract]
Gao Y, Woo KC. 1996. Regulation of phosphoenolpyruvate carboxylase in Zea mays by protein phosphorylation and metabolites and their roles in photosynthesis. Australian Journal of Plant Physiology 23, 25–32.
Häusler RE, Bailey KJ, Lea PJ, Leegood RC. 1996. Control of photosynthesis in barley mutants with reduced activities of glutamine synthetase and glutamate synthase. III. Aspects of glyoxylate metabolism and effects of glyoxylate on the activation state of ribulose-1,5-bisphosphate carboxylase-oxygenase. Planta 200, 388–396.[Web of Science]
Huber SC, Huber JL, McMichael RW. 1994. Control of plant enzyme activity by reversible protein phosphorylation. International Review of Cytology 149, 47–98.
Jenkins CLD, Furbank RT, Hatch MD. 1989. Inorganic carbon diffusion between C4 mesophyll and bundle sheath cells. Direct bundle sheath CO2 assimilaton in intact leaves in the presence of an inhibitor of the C4 pathway. Plant Physiology 91, 1356–1363.
Kacser H, Burns JA. 1973. The control of flux. Symposium of the Society for Experimental Biology 27, 65–104.
Leegood RC, von Caemmerer S. 1988. The relationship between contents of photosynthetic metabolites and the rate of photosynthetic carbon assimilation in leaves of Amaranthus edulis L. Planta 174, 253–262.
Leegood RC, von Caemmerer S. 1989. Some relationships between contents of photosynthetic intermediates and the rate of photosynthetic carbon asimilation in leaves of Zea mays L. Planta 178, 258–266.
Leegood RC, von Caemmerer S. 1994. Regulation of photosynthetic carbon assimilation in leaves of C3–C4 intermediate species of Moricandia and Flaveria. Planta 192, 232–238.
Leegood RC, von Caemmerer S, Osmond CB. 1997. The flux of metabolites in C4 and CAM plants. In: Dennis DT, Turpin DH, Lefebvre DD, Layzell DB, eds. Plant metabolism. London: Longman, 274–298.
Leegood RC, Walker DA, Foyer CH. 1985. Regulation of the Benson–Calvin cycle. In: Barber J, Baker NR, eds. Photosynthetic mechanisms and the environment, Vol. 6. Amsterdam: Elsevier, 189–258.
Lichtenthaler HK, Wellburn AR. 1983. Determinations of total carotenoids and chlorophylls a and b of leaf extracts in different solvents. Biochemical Society Transactions 11, 591–592.
Lowry OH, Passonneau JV. 1972. A flexible system of enzymatic analysis. New York: Academic Press.
McNaughton GAL, MacKintosh C, Fewson CA, Wilkins MB, Nimmo HG. 1991. Illumination increases the phosphorylation state of maize leaf phosphoenolpyruvate carboxylase by causing an increase in the activity of protein-kinase. Biochimica et Biophysica Acta 1093, 189–195.[Medline]
O'Leary MH. 1982. Phosphoenolpyruvate carboxylase: an enzymologist's view. Annual Review of Plant Physiology 33, 297–315.[Web of Science]
Paul MJ, Knight JS, Habash D, Parry MAJ, Lawlor DW, Barnes SA, Loynes A, Gray JC. 1995. Reduction in phosphoribulokinase activity by antisense RNA in transgenic tobacco: effect on CO2 assimilation and growth in low irradiance. The Plant Journal 7, 535–542.[Web of Science]
Quick WP. 1994. Analysis of transgenic tobacco plants containing varying amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase. Biochemical Society Transactions 22, 899–903.[Web of Science][Medline]
Stitt M. 1995. Metabolic regulation of photosynthesis. In: Baker NR ed. Photosynthesis and the environment. Dordrecht: Kluwer, 151–190.
Ting IP, Osmond CB. 1973. Photosynthetic phosphoenolpyruvate carboxylases. Characteristics of alloenzymes from C3 and C4 plants. Plant Physiology 51, 439–447.
Trevanion SJ, Furbank RT, Ashton AR. 1997. NADP malate dehydrogenase in the C4 plant F. bidentis: cosense suppression of activity in mesophyll and bundle sheath cells and consequences for photosynthesis. Plant Physiology 113, 1153–1165.[Abstract]
Uedan K, Sugiyama T. 1976. Purification and characterization of phosphoenolpyruvate carboxylase from maize leaves. Plant Physiology 57, 906–910.
Urbina JA, Avilan L. 1989. The kinetic mechanism of phosphoenolpyruvate carboxykinase from Panicum maximum. Phytochemistry 28, 1349–1353.
von Caemmerer S, Farquhar GD. 1981. Some relationships between the biochemistry of photosynthesis and the gas exchange of leaves. Planta 153, 376–387.[Web of Science]
von Caemmerer S, Furbank RT. 1999. Modelling C4 photosythesis. In: Sage RF, Monson RK, eds. C4 plant biology. New York: Academic Press, 173–211.
Walker RP, Acheson RM, Técsi LI, Leegood RC. 1997. Phosphoenolpyruvate carboxykinase in C4 plants: Its role and regulation. Australian Journal of Plant Physiology 24, 459–468.
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  H. Bauwe and U. Kolukisaoglu Genetic manipulation of glycine decarboxylation J. Exp. Bot., June 1, 2003; 54(387): 1523 - 1535. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||